As semiconductor fabrication technology advances, the size features formed into semiconductor devices decreases. This makes it more important to control the critical dimensions of the features formed into a semiconductor wafer. Small changes in the critical dimension can cause unacceptable changes to the semiconductor components. Thus, measurement and control techniques are very important to efficient semiconductor fabrication.
Methods for measuring features formed into a semiconductor substrate include optical tools that utilize scatterometry. Scatterometry refers to the process of measuring light spectra to determine the size of periodic structures. Scatterometry can provide information such as pitch, sidewall angles, and line width. Another tool that can be used to measure characteristics of features formed into semiconductor substrates is a scanning electron microscope.
Various conditions may affect the characteristics of features on a substrate. For example, the intensity of light used to expose a photoresist layer affects the characteristics of the feature. The level of intensity of light may be referred to as dosage or exposure. Another condition that affects the characteristics of features is the focus of the photolithography tool. The photolithography tool can be set to focus at varying depths into a photoresist layer.
The combination of both focus and exposure has a significant effect on the overall characteristics of features formed into a substrate. Moreover, the focus and exposure settings of a photolithography tool may drift over time. Thus, it is useful to monitor this drift and make adjustments as necessary to ensure the formation of features with desired characteristics. One effective means of monitoring and controlling the semiconductor fabrication process is to model the relationships between measured characteristics and focus and exposure settings. It is therefore desirable to find effective methods of modeling these relationships.